The present invention relates to a sheet supplying device for supplying a sheet from a sheet cassette to a sheet feed path of an image forming apparatus such as a copying machine, and more particularly to a sheet supplying device for supplying a developer sheet from a sheet cassette in an image forming apparatus which employs a photosensitive pressure-sensitive recording sheet for carrying a latent image thereon and transferring a developed image to the developer sheet.
There has been recently utilized an image forming apparatus such as a copying machine employing a continuous photosensitive recording medium such as a microcapsule sheet and a developer sheet used in combination with the photosensitive recording medium. In the image forming apparatus as described above, plural developer sheets are stacked in a sheet cassette and are supplied one by one from the sheet cassette by suction means having a suction cup for at tracting a sheet to the suction cup under a negative pressure and feeding the attracted sheet through a sheet feed path toward a pressure-developing unit. A sheet supplying device for performing a sheet supplying operation with the suction means is disclosed in detail in Japanese Unexamined Published Patent Application No. 55-93744.
The image forming apparatus using the sheet supplying device as described above is provided with various sensors for detecting operational conditions for performing a series of copying processes. For example, there is a sheet detection sensor for detecting whether the sheet supplied from the sheet cassette is present in a sheet feed path between the sheet cassette and the pressure-developing unit to thereby determine whether a sheet supply error occurs, a photosensor for detecting a light-amount of the halogen lamp 5 of the light source, an uppermost sheet sensor for detecting the uppermost sheet of the developer sheets stacked in the sheet cassette 17 and so on.
The image forming apparatus as described above is further provided with a suction means moving mechanism for mechanically moving the suction means upwardly and downwardly and supplying the developer sheet attached to the suction cup to the sheet feed path every sheet supply operation.
On the other hand, the suction means having a suction cup is required to be mechanically moved upwardly and downwardly to supply a sheet attached to the suction cup to the sheet feed path, and therefore detection of a sheet supply error and a processing for the sheet supply error such as a ceasing operation of the suction means, a ceasing operation of a sheet feed operation of the microcapsule sheet and so on cannot be performed at an early stage after the error is detected. Accordingly, there are cases where the photosensitive pressure-sensitive recording sheet is vainly fed in the sheet feed path and an unnecessary operation is carried out irrespective of occurrence of the error.
In the image forming apparatus thus constructed, when an error such as a sheet supply error, an exposure error or the like occurs in a sheet supply operation of the sheet supplying device as described above, the sheet supplying operation is required to be halfway ceased. In this case, the driving of the suction means moving mechanism is stopped simultaneously with the detection of the error by the sensor, and therefore the following troubles are liable to occur.
Since an operation of moving the suction means by the suction means moving mechanism is ceased at the time when the sheet supply error is detected by the sheet detection sensor, there are possibilities that the suction means are stopped at its descent position and that the suction means are stopped while the sheet is attached to the suction cup. In the former case, when the sheet cassette is removed from the apparatus to remove a sheet in trouble from the sheet feed path, the suction means is abruptly contacted with the sheet cassette and the sheet being removed and is damaged thereby. Further, in the latter case, it is difficult to separate the sheet from the suction cup because the sheet is attached to the suction cup with a vacuum force, and the suction cup is liable to suffer damage when the sheet is forcedly removed from the suction cup. These disadvantages cause durability of the sheet supplying device to be lowered.
Still further, in the image forming apparatus thus constructed, when the sheet supply operation is halfway ceased due to occurrence of an operational error or cut-off of the power supply and the sheet supplied from the sheet cassette is stopped at a position in the sheet feed path where the leading end of the sheet does not reach the sheet detection sensor, the sheet in the sheet feed path cannot be detected by the sheet detection sensor when the sheet supply operation is resumed, and therefore a sheet jam is not detected although the sheet jam actually occurs in the sheet feed path.